The conventional ultrasonic flowmeters for measuring a flow rate of a fluid from the difference in ultrasonic wave propagation time are generally classified into three types.
FIG. 9 shows a structure of an ultrasonic flowmeter according to a first prior art. In FIG. 9, reference numeral “100” designates a substantially U-shaped flow passage-forming body through which a fluid flows as indicated by solid arrows. Reference numerals “101” and “102” designate ultrasonic transducers, which are arranged on both sides of a linear pipe portion 103 of the flow passage-forming body 100. In this ultrasonic flowmeter, when the ultrasonic transducer 101 on the upstream side is excited by an electrical signal from a converter (not shown) while the flow passage-forming body 100 is filled with the fluid flowing therein, an ultrasonic wave is generated and propagated through the fluid in the straight pipe portion 103 of the flow passage-forming body 100. The ultrasonic wave is received by the ultrasonic transducer 102 on the downstream side and converted into an electrical signal, which is output to the converter. After that, the ultrasonic transducer 102 on the downstream side is instantaneously excited by the electrical signal from the converter. The ultrasonic wave thus generated is propagated through the fluid in the straight pipe portion 103, received by the ultrasonic transducer 101 on upstream side, and converted into an electrical signal, which is output to the converter. In the process, the difference of the ultrasonic wave propagation time is used to determine the velocity of the fluid in the flow passage-forming body 100 and to measure the flow rate (as disclosed, for example, in Japanese Unexamined Patent Publication No. 2000-146645).
FIG. 10 shows a structure of an ultrasonic flowmeter according to a second prior art. In FIG. 10, reference numeral “110” designates a measurement pipe in which a fluid flows as indicated by solid arrows. Reference numerals “111” and “112” designate detectors which are, in pairs, clamped at positions opposed to each other in different annular lines on the outer peripheral surface of the measurement pipe 6. In this ultrasonic flowmeter, the ultrasonic vibration generated from the detector 111 is propagated diagonally with respect to a direction of flow of the fluid in the measurement pipe (in a direction indicated by dashed arrows in FIG. 10) and received by the detector 112. In this case, as in the first prior art, the ultrasonic flowmeter is one in which the transmitting and receiving of the detectors 111, 112 are switched to measure the flow rate from the difference in the propagation time of the ultrasonic vibrations. The structure of the detectors 111, 112 in FIG. 10 is shown diagrammatically in FIG. 10 and, in actual installation as described in Japanese Unexamined Patent Publication No. 2003-75219, the ultrasonic transducer is mounted on an inclined surface of a wedge-shaped fixing device so that the ultrasonic wave from the ultrasonic transducer may be propagated diagonally with respect to the center axis of the pipe.
FIG. 11 shows a third prior art. As shown in FIG. 11, two detectors 114, 115 are clamped in alignment, and in spaced relation to each other, on the outer peripheral surface of a measurement pipe 113. In this measurement pipe 113, the ultrasonic vibration, generated by the detector 114 having an ultrasonic transducer similar to that of the second prior art, is reflected in the direction of dashed arrows of FIG. 11 on the inner surface of the measurement pipe 113, so that the flow rate is measured from the difference in the propagation time of the ultrasonic vibration between the case in which the reflected ultrasonic vibration is received by the detector 115 and the case in which the ultrasonic vibration generated from the detector 115 is reflected on the inner surface of the measurement pipe 113 and received by the detector 114.
However, the ultrasonic flowmeter according to the first prior art has a substantially U-shaped flow passage-forming body 100. This causes a problem that, in the case where the fluid flowing in the flow passage-forming body 100 is a slurry, the slurry tends to be deposited and fixed on the curved portions 104 of the flow passage-forming body 100 and the propagation of the ultrasonic vibration is hampered thereby making accurate flow rate measurement impossible especially in a CMP (chemical mechanical polishing) device in the semiconductor field. Also, a problem is posed that the curved portions 104 of the flow passage-forming body 110 cause a pressure loss of the fluid in the flow passage-forming body 110, thereby making it impossible to accurately measure the velocity and hence the flow rate. Further, a problem is posed that the substantially U-shaped pipe path results in a high production cost.
In the ultrasonic flowmeter according to the second prior art, while the slurry is not deposited, a smaller bore of the measurement pipe is required to obtain a measurable velocity in the measurement of a micro flow rate. This correspondingly reduces the mounting distance between the detectors 111 and 112, thereby resulting in a problem that the resulting smaller propagation distance and propagation time difference make an accurate measurement, or any measurement, impossible. Further, the fixing devices 111 and 112 are used for efficient propagation of the ultrasonic vibration diagonally with respect to the axial direction of the pipe, and the intervention of a material, such as epoxy resin, lower in propagation rate than the measurement pipe is required to reduce the reflection from the measurement pipe as a method of improving the measurement sensitivity. However, the use of the resin alone has a disadvantage of an increased attenuation of the ultrasonic vibration.
In addition, the third prior art has problems that the reflection increases the attenuation of the ultrasonic vibration and thereby makes it difficult to measure a micro flow rate and that the mounting of the detectors 114 and 115 is difficult.